The composition of rain water represents a significant indication of atmospheric contaminants because rain carries such contaminants to the ground.
From ecological studies it is known that substances soluble in rain water are carried to the soil and are picked up by the plant cover, such contaminants including, for example, toxic materials such as heavy metals in solubilized form.
Such contaminants may be released into the atmosphere in the form of dust or aerosols and are picked up by the rain water by mechanical entrainment or solubilization and are carried therewith to the ground. The systematic monitoring of rain water in the environment for potentially toxic, noxious, poisonous or other detrimental materials is of advantage in determining the source of these pollutants and is important in maintaining an environment substantially free therefrom.
To this end, samples are taken of rain water and are analyzed for their impurity contents, generally with the aid of sampling vessels open upwardly and placed at selected locations. The rain water collects in these vessels and then is removed therefrom for analysis.
It has been recognized that it is of advantage to put out such rain water collectors or sampling vessels just before the beginning of a rainfall to avoid contamination of the vessel with dust particles which may otherwise enter the vessel or accumulate therein. However, this poses a problem since weather is often unpredictable and it frequently is not possible to provide operating personnel who can put out the requisite collecting vessel just before the rainstorm in every instance.
Furthermore, it is frequently desired to collect rain water in dependence upon wind strength of direction because the contaminants from a particular direction may afford a greater level of useful information than rain water collected when the wind is blowing from another direction. This has posed an additional stricture upon the individuals responsible for placing the sampling or collecting vessels.
In fact, with existing methods of rain water collection, considerable monitoring of the weather, wind direction and like parameters is required by alert individuals who must always be available or present and frequently, apart from the high cost of operating in this manner, the collection is unsatisfactory.
Note should be taken of the fact that the mean-value results which can be obtained from existing meteorological stations often do not satisfy the requirements as to particularity of collection from rain with the wind blowing from a certain direction. As a practical matter, at least for determining the source of certain types of contamination, it is vital to make the sample collection dependent upon the instantaneous wind speed and the instantaneous wind direction.